As a shock absorber to be built into a suspension system for vehicles, there is known a shock absorber including a separator tube arranged between a cylinder and an outer tube. For example, in a shock absorber disclosed in Patent Literature 1, the separator tube is fitted to an outer periphery of the cylinder, and a space between both radially shrunk end portions of the separator tube and the cylinder is sealed by sealing rings. Further, sealing ring grooves (housings) each having a substantially quadrangular shape in cross-section and extending in a circumferential direction are formed along an inner periphery of both the end portions of the separator tube. The sealing ring grooves can be formed, for example, by a beading process to be executed on the separator tube having a cylindrical shape.
Incidentally, the beading process refers to a sequential rotational process to be executed through rotation and revolution of a roller die. Thus, a material of the end portions of the separator tube is caused to plastically flow both in the circumferential direction and an axial direction. As a result, acute raised portions are formed along the circumferential direction at groove corners of each of the sealing ring grooves. In this case, the acute raised portions are ignorable when a gap (clearance) between the cylinder and the separator tube is substantially zero. However, in consideration of assembly efficiency, a predetermined gap is secured between the cylinder and the separator tube. Thus, when the shock absorber is in such a state that an internal pressure repeatedly varies intensively on a compression side or a decompression side, the sealing rings are repeatedly protruded slightly from the sealing ring grooves and restored thereto. In this configuration, when backup rings are not used, the slightly protruded portions of the sealing rings repeatedly slide against the acute raised portions. As a result, the sealing rings may be damaged.